The present invention relates to a signal analyzing apparatus for measuring frequency characteristics of a signal employed for a mobile communication system such as automobile telephone or portable telephone, and displaying a waveform of the signal, thereby analyzing the signal.
A signal employed for a mobile communication system such as automobile telephone or portable telephone, for example, is modulated by a variety of systems.
In addition, a TDMA (Time Division Multiple Access) system is employed as a communication system in order to efficiently use a communication line.
A frequency of a carrier wave for carrying a signal employed in such a mobile communication system ranges some hundreds MHz to some GHz, which is very high.
In general, a signal analyzing apparatus such as spectrum analyzer is employed for precisely measuring a variety of frequency components included in such a signal.
FIG. 3 is a block diagram depicting a general configuration of a conventional signal analyzing apparatus used for measuring frequency characteristics of a measured signal with its high frequency.
In a signal analyzing apparatus 21 shown in FIG. 3, a measured signal with its high frequency inputted via an input terminal 22 is adjusted to a predetermined, normalized level by an attenuator (ATT) (not shown).
Then, the level adjusted, measured signal with its high frequency is mixed with a local oscillation signal from a local oscillator 24 by means of a signal mixer 23, and the mixed signal is converted into an intermediate frequency signal having its intermediate frequency.
Here, the oscillation frequency of the local oscillator can be swept (frequency swept) over the range of predetermined frequencies by means of a sweep control section (not shown).
In this manner, a frequency of the intermediate frequency signal outputted from the signal mixer 23 also changes in synchronization with a sweep operation.
Then, the intermediate frequency signal with its reduced frequency is inputted to a resolution bandwidth (hereinafter, referred to as RBW) filter 25, an undesired frequency component is eliminated by means of the RBW filter 25, and only a required intermediate frequency signal is selected.
A bandwidth (RBW) at a time when a peak level at the passage center frequency of the frequency characteristics of this RBW filter 25 drops by 3 dB indicates a frequency resolution in this signal analyzing apparatus.
A signal from the RBW filter 25 is gain adjusted by means of an amplifier (not shown), and a switching section 26 is switched to a LOG converter 27 side. In this state, a signal logarithm converted by means of a LOG converter 27 to be compressed is detected by means of a waveform detector (DET) 28.
In contrast, when the switching section 26 is switched to the RBW filter 25 side, the signal from the RBW filter 25 is detected by means of a waveform detector (DET) 28.
The signal detected by this waveform detector 28 within a sweeping period indicates the size of a time series waveform at the swept frequency.
The thus outputted signal by the waveform detector 28 is inputted to an anti-aliasing filter 29.
The anti-aliasing filter 29 used here is composed of a filter for eliminating a high frequency component (noise component) of a frequency spectrum waveform finally displayed at a display section 34 provided at a panel of an apparatus main body.
The signal from this anti-aliasing filter 29 is converted into digital data by means of a next A/D converter 30, and the converted digital data is stored in a data storage section 31.
Predetermined processing is applied to the digital data stored in this data storage section 31 by means of a signal processing section 33.
Then, the frequency spectrum waveform obtained by this processing is displayed in a frequency domain (frequency on horizontal axis and amplitude on vertical axis) on a display screen of the display section 34.
In the meantime, in the signal analyzing apparatus 21 of such type, a signal employed in a mobile communication system such as automobile telephone or portable telephone, the signal being inputted as a measured signal is a burst shaped signal whose level changes with an elapse of time.
In the field of such mobile communication system, there is a demand to measure such burst shaped signal in detail by tracking a time.
The signal analyzing apparatus 21 shown in FIG. 3 is provided with a function for performing time span sweeping such that a frequency of the local oscillator 24 is fixed so as to measure a time change of a signal bandwidth-restricted by the RBW filter 25 within a normalized bandwidth, thereby displaying the result of the time span sweeping while time and amplitude are defined on the horizontal and vertical axes, respectively, on the display screen of the display section 34.
By this time span sweeping, in the case where a burst shaped measured signal is measured in detail by tacking a time, there have been conventionally employed a method of measuring the signal by changing a sampling rate of an A/D converter and a method of decimating unwanted data after sampling has been performed at a sufficiently high speed by employing an A/D converter that operates at a high speed.
However, in the method of changing the sampling rate of the A/D converter, it has been necessary to reacquire data every time the sampling rate is changed.
Moreover, in the case where the sampling rate is changed, thereby causing operation at a high speed, there has been a problem that a sufficient dynamic range cannot be obtained.
In the method of decimating unwanted data after sampling has been performed at a sufficiently high speed by using the A/D converter that operates at a high speed, it has been necessary to use a sampling rate of the lowest common multiple for the resolution of data per one time domain to be acquired.
For example, in the case where 1 xcexcsec is required as a time span, assuming that 500 items of data are provided, a resolution of 2 nsec per one item of data is obtained. Thus, the sampling rate of the A/D converter is set to a frequency of 500 MHz.
Similarly, the sampling rate of the A/D converter at a resolution of 5 nsec is set to a frequency of 200 MHz.
In order to meet resolutions of both of the above 2 nsec and 5 nsec, it is required that the A/D converter operates when the sampling rate of the converter is set to a frequency of 1 GHz.
Therefore, with the above described method, the sampling rate of the A/D converter could not be changed freely.
Even if the sampling rate can be changed, in the case of a high speed operation, there has been a problem that a sufficient dynamic range cannot be obtained.
In addition, there has been a problem that a memory requires its capacity corresponding to the maximum operation.
Namely, in the case where the sampling rate is changed, thereby causing high speed operation, it is required to use an A/D converter that corresponds to the highest speed operation. In the A/D converter that corresponds to high speed operation, there has been a problem that a sufficient conversion bit cannot be allocated, processing must be done at the same conversion bit even during a low speed sampling, and there is a limitation to a dynamic range according to the conversion bit, thus making it impossible to obtain a sufficient dynamic range.
In the meantime, in the signal analyzing apparatus 21 shown in FIG. 3, a signal bandwidth-limited by the RBW filter 25, the signal passing through the waveform detector 28, is a base band signal having a bandwidth of the RBW filter 25.
The inventors found that the bandwidth of the RBW filter 25 is sampling at a sampling rate that can be reproduced, and then, arbitrary time data is generated by means of re-sampling using a digital signal processing technique, whereby detailed time analysis can be performed without changing the sampling rate, and reached the present invention based on the findings.
The present invention has been made in order to solve the foregoing problems. It is an object of the present invention to provide a signal analyzing apparatus capable of performing detailed time analysis by reproducing arbitrary time data without increasing a sampling rate of an A/D converter, and capable of obtaining a sufficient dynamic range.
In order to achieve the foregoing object, according to a first aspect of the present invention, there is provided a signal analyzing apparatus comprising:
a resolution bandwidth (hereinafter, referred to as RBW) filter 5 in which a bandwidth is set so as to selectively pass a frequency component of only a desired signal bandwidth, of the measured signal frequency-converted into a normalized intermediate frequency signal;
a waveform detector 8 for detecting a signal passing through the RBW filter;
an analog/digital (hereinafter, referred to as A/D) converter 10 for sampling the signal detected by the waveform detector at a predetermined sampling rate at which a Nyquist frequency is within the frequency bandwidth of the RBW filter, thereby converting the sampled signal into digital data;
a data storage section 11 for storing digital data converted by the A/D converter;
a signal processing section 13 for re-sampling the digital data stored in the data storage section so as to enable to reproduce a bandwidth of the signal detected by the waveform detector, thereby generating arbitrary time data; and
a display section 34 for displaying the arbitrary time data generated by the signal processing section while time and amplitude are defined on horizontal and vertical axes, respectively, on a display screen.
In addition, in order to achieve the foregoing object, according to a second aspect of the present invention, there is provided a signal analyzing apparatus according to the first aspect, wherein re-sampling at the signal processing section is performed by using at least one of line interpolation, spline function interpolation and sampling function interpolation.
Further, in order to achieve the foregoing object, according to a third aspect of the present invention, there is provided a signal analyzing apparatus according to the second aspect, wherein re-sampling at the signal processing section is performed by using the sampling function interpolation, and a passing bandwidth of the sampling function interpolation is limited by a window function.
Furthermore, in order to achieve the foregoing object, according to a fourth aspect of the present invention, there is provided a signal analyzing apparatus according to the first aspect, wherein an anti-aliasing filter 9 set in a passing bandwidth encompassing the maximum bandwidth of the RBW filter is provided between the RBW filter and the A/D converter.
Still furthermore, in order to achieve the foregoing object, according to a fifth aspect of the present invention, there is provided a signal analyzing apparatus according to the first aspect, wherein the signal processing section comprises:
data acquisition management means for, in interpolating data between the existing data, guaranteeing acquisition of interpolation data before and after generation data used for interpolation, and then, associating a data acquisition timing from the data storage section with an address of the data storage section;
processing condition converting means for determining a condition corresponding to a data generation resolution (time span) indicating how many address in the data storage section is required for one item of data, and determining a condition corresponding to a data acquisition timing indicating what is the number of data generated in the data storage section or indicating the number of address from which the data in the data storage section must be used;
data generating means for using a re-sampling function (or interpolation function and decimation), thereby generating data between the existing sampling data stored in the data storage section by means of re-sampling; and
display control means for controlling a display section so as to display a level variation of the measured signal based on data generated by the data generating means based on the data stored in the data storage section while time and amplitude are defined on horizontal and vertical axes, respectively, on the display screen of the display section.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.